Fluid filled devices are devices that contain at least two fluids (i.e. multi-fluid filled devices), with the device typically being arranged to perform a function by displacing (changing the position or shape of) a volume of at least one of the fluids.
Optical fluid filled devices can for instance function as lenses, diaphragms, gratings, shutters, optical switches or filters. Examples of optical fluid filled devices, as well as different possible methods of displacing the fluids such as by using electrowetting, are described within WO 02/069016.
Electrowetting devices are devices that utilise the electrowetting phenomenon to operate. In electrowetting, the three-phase contact angle is changed with applied voltage. The three-phases constitute two fluids and a solid. The term fluid encompasses both liquids and gases. Typically, at least the first fluid is a liquid; the second fluid may be a liquid, or a gas or vapour.
EP 1,069,450 describes an optical device that utilises the electrowetting effect so as to act as a variable density optical filter. FIG. 1 is a cross-sectional view of such a typical optical device 90. The optical device 90 has two immiscible fluids 80, 87 confined in a sealed space 92, (i.e. a chamber, or cavity). The term immiscible indicates that the two fluids do not mix. The first fluid 80 is an insulator (e.g. silicone oil) and the second fluid 87 electroconductive (e.g. a mixture of water and ethyl alcohol). The first fluid 80 and the second fluid 87 have different light transmittances.
A voltage from a voltage supply 40 can be applied to the two electrodes 41, 42 so as to produce an electric field between the fluid 87 and the electrode 42 (an insulating layer 50 prevents the second electrode 42 contacting the conductive second fluid).
By varying the voltage applied to the second fluid 87, the shape of an interface 85 between the first fluid 80 and the second fluid 87 is altered, so as to change the overall transmittance of the optical element. It is also known to provide a variable lens utilising a similar configuration, but with the two fluids 80, 87 having different refractive indices.
The device 90 in FIG. 1 has a water-repellent film 60 of diameter D1 on the insulating layer 50, surrounded by a ring of a hydrophilic agent 70 so as to locate the first fluid 20. The shape of the interface 85 changes during the operation of the device. The change in shape may result in the fluid 80 extending from the water-repellent layer 60 to the opposite surface of the sealed space. In order to prevent the first fluid 80 adhering to the opposite surface, a portion of the opposite surface is coated with a layer of hydrophilic film 72 of diameter D2.
In order to limit the action of gravity upon the interface 25, the two fluids 80, 87 may be of equal density.
It is an aim of embodiments of the present invention to provide an improved electrowetting device. It is an aim of embodiments of the present invention to provide an electrowetting device that has improved stability, particularly when subjected to accelerative forces.